Speckle is a phenomenon that affects all lasers and laser line projectors. It is caused by the roughness of the surface that is illuminated causing tiny diffractive regions that give the surface a grainy random “speckle” appearance. The speckle effect is a result of the interference of many waves of the same frequency, having different phases and amplitudes, which add together coherently to give a resultant wave whose amplitude, and therefore intensity, varies randomly. When a surface is illuminated by a light wave, according to diffraction theory, each point on an illuminated surface acts as a source of secondary spherical waves. The light at any point in the scattered light field is made up of waves, which have been scattered from each point on the illuminated surface. If the surface is rough enough to create path-length differences exceeding one wavelength, giving rise to phase changes greater than 2n, the amplitude, and hence the intensity, of the resultant light varies randomly. If light of low coherence (i.e., made up of many wavelengths) is used, a speckle pattern will not normally be observed, because the speckle patterns produced by individual wavelengths have different dimensions and will normally average one another out. However, speckle patterns are inherent in coherent light sources, such as lasers.
Speckle can be problematic in certain imaging applications—for example a laser displacement sensor (DS), which projects a laser line (based on a fan formed (e.g.) by passing a laser beam through an aspherical lens (such as a Powell lens or another appropriate generator including cylinder lenses, holographic, cylinder arrays, linear diffusers, or combinations thereof) onto a surface, and receives the reflected light at a camera sensor along a camera axis that is not parallel to the axis of the laser fan. The DS processor thereby triangulates on the surface profile. However, the lumpy and asymmetric irregularity of the received line light, as a result of speckle, limits the precision of the location of the line by the camera sensor. The lumpiness in the line is read as an error in height of the surface. The reduction of speckle enables a lower uncertainty and higher accuracy measurement of profile by the DS.
There are several available techniques for reducing speckle. For example, a moving diffuser can be placed within the path of a stationary beam, causing an incoherent superposition of random speckles fills in the image of the line with the average of several uncorrelated speckle patterns. Disadvantageously, the diffuser tends to be a relatively high in mass (compared to the scale of other components), and moving it requires mechanical complexity and can limit the rate of change of the speckle pattern, and hence, the frame rate of the system. Other approaches for reducing speckle (including wavelength broadening) are also disadvantageous to varying extents, involving added cost, complexity and/or other disadvantages. Thus, such approaches are less desirable to employ in a practical laser line-projecting arrangement.
It can also be challenging to accurately scan an object using a laser line. In general, many scanning arrangements rely upon the object, the camera and/or the illuminator to move as motion is tracked and translated into relative distance within the vision system processor. This requires mechanical systems that can be subject to wear and degradation due to (e.g.) conditions in the scanning environment.